Spring Conception Ups Risk of Preterm Birth

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Babies conceived in the spring may be at increased risk of being
born prematurely, a new study suggests.

The results show that the rate of
premature birth is about 10 percent higher among babies
conceived in May, compared with those conceived in other months.

This finding may be related to the seasonal pattern of the flu
virus, the researchers said. Babies conceived in May are due in
February, which is typically the height of flu season.

"Because influenza is known to cause premature labor, these
infants are at higher risk of short gestation," the researchers
write in the July 8 issue of the journal Proceedings of the
National Academy of Sciences.

In fact, there was a strong link in the study between the
prevalence of flu and the risk of preterm birth. During the H1N1
flu pandemic in 2009, when the flu
season started early, the highest risk for preterm birth
shifted to babies conceived in February and March. [See
7 Ways Pregnant Women Affect Babies ].

The findings suggest that flu shots might be effective at
preventing seasonal increases in preterm birth, the researchers
said. In the United States, doctors already recommend
flu shots for pregnant women, who are at increased risk for
complications from the flu.

The study also found a link between a baby's month of conception
and his or her weight at birth. Babies conceived during the
summer months tended to weight about 8 to 9 grams (0.28 to 0.32
ounces) more than babies conceived at other times of the year.
This increase in birth weight may be tied to the mother's
weight gain in pregnancy.

"Women gain almost one pound more [in pregnancy] when they
conceive in June, July or August than when they conceive in
January," the researchers said.

This finding suggests that seasonal patterns in food consumption
may affect birth outcomes, even in developed countries, the
researchers said.

Previous studies have linked a child's month of conception with a
slew of outcomes, including IQ and risk for neurological
disorders. However, in many of these studies, it was possible
that differences between mothers, such as level of income, may
have be the real reason for the link, rather than month of
conception.

But in the new study, the researchers accounted for this
possibility by comparing siblings born to the same mother. The
study examined information from 1.4 million children born to
647,050 mothers in New York, New Jersey and Pennsylvania.

However, the researchers said they still could not investigate
all potential reasons why month of birth may be related to
preterm birth and birth weight; seasonal allergies and changes in
temperature may also play a role, the researchers said.